Filtration systems which employ cartridge filters employ a porous filter material through which the gaseous medium passes, the solids being retained on the upstream surface of the filter medium as the gases pass through. The separated solids build up on the upstream side of the filter medium with time and must be removed before the thickness of the accumulated solids layer causes excessive pressure drop. In current practice, the filter cake is dislodged from the filter elements by means of a pulse of compressed air admitted to the filter element, a reverse air flow, or a shaker mechanism. The filter medium flexes under the force of the air pulse to dislodge the filter cake and the flow of gases being filtered is maintained while such flexing and dislodging is taking place.
Cartridge filters generally have a folded or pleated filter medium arranged cylindrically around a perforated cylindrical supporting member or tube. The supporting tubes and filter medium are adhered to end caps which prevent leakage around the filter medium and provide support for the filter medium.
For the most part, felts and fabrics have been used as the filter media with the addition of paper and other nonwoven backings. Cartridges of these types typically pack pleats very close together, e.g. 7-12 pleats per inch, for two reasons: (1) to maximize filter media surface area, and (2) to improve physical strength of the thin filter media where each pleat is unsupported to the cartridge during operation and cleaning.
An example is found in U.S. Pat. No. 4,878,930 Manniso, et al. which uses an expanded porous polytetrafluoroethylene (ePTFE) membrane laminated to a felt or fabric and which is supported by a wire backing. The pleat spacing is greater than typically used with other filter media due to the fact that ePTFE membranes are more easily cleaned. On the cleaning cycle, it is important to leave enough space to allow dust (dirt. product, etc.) to exit the pleat space. This is the reason why ePTFE membrane filter cartridges are ruggedly supported, to allow the spacing for efficient cleaning while supporting the relatively open pleat that could not be supported by conventional means. It was, therefore, felt that a lightweight media would have to have tightly packed pleats to obtain support during cleaning and thereby avoid blow-out failure. The tight pleat needed to support such a cartridge is very ineficient in the cleaning cycle; thus, this leads to product build-up and high pressure drops. A more open pleat does not work well because high pressure pulsed air needed to clean that filter would blow out the cartridge.
It was previously believed that the use of ePTFE laminated to a nonwoven would have to be manufactured in the conventional way to give support to the media in the form of stays or the like. It was further reasoned that a tight pleat was needed for support would minimize the effectiveness of the cleanable ePTFE laminate.